Fuel management systems for aircraft gas turbine engines control both fuel mass flow rate and pressure, maintaining demanded levels of engine performance over a broad range of operating conditions and environments. Should this range be exceeded, and to account for unforeseeable circumstances, these systems usually offer the capability to override some or all of these automatic controls. Typically, the most fundamental of these override safeguards is the one providing for shut down of all fuel flow to the engine injectors.
In most present systems, a shutoff valve located in the engine fuel line controls flow to the engine injectors. When the pressure applied at an actuation port located on this shutoff valve exceeds some predetermined pressure, the valve closes and all fuel flow stops. Conversely, when the pressure at the actuation port on the shutoff valve falls below this predetermined pressure, the valve opens. To control its pressure, the actuation port connects to an outlet of a sequencing valve having two inlets and two control ports. The first inlet connects to a fuel source with sufficient pressure to exceed the predetermined shutoff valve actuation pressure. The second inlet connects to the system drain, a low pressure source.
When high pressure is applied at the first control port, or run port, the outlet connects with the second inlet, venting the actuation port to the system drain and opening the shutoff valve. When the outlet connects to the second inlet in this fashion, a hydraulic latch inside the sequencing valve engages. With this latch engaged, the outlet will remain connected to the second inlet, regardless of the pressure applied at the run port. To disengage this latch, high pressure is applied at the second, or shutdown, port. When the latch disengages, the outlet connects to the first inlet, pressurizing the actuation port to close the shutoff valve.
To control the pressure at the run and shutdown ports, each connects to a two-position solenoid valve. When the shutdown solenoid valve energizes, high pressure is applied at the shutdown port. When the shutdown solenoid valve is de-energized, the shutdown port vents to the system drain. At all times, the run port vents, through an orifice in a vent line, directly to the system drain. When the run solenoid valve is energized, high pressure fuel is delivered to the run port. The size of the orifice in the vent line is selected to allow the high pressure fuel to flow into the system drain at a low rate. When the run solenoid subsequently de-energizes, high pressure fuel in the line supplying the run port bleeds to the drain through the vent line, venting the run port.
Before the engine fueled by the system is started, both the shutdown and the run solenoid valves remain de-energized. As both ports on the sequencing valve are therefore vented, high pressure fuel is delivered to the actuation port, keeping the shutoff valve closed. When the engine is to be started, the run solenoid valve energizes, pressurizing the run port. The sequencing valve then latches, opening the shutoff valve by venting the actuation port to the system drain. When the running engines are to be shut down, the shutdown solenoid valve energizes, pressurizing the shutdown port to disengage the hydraulic latch. With the hydraulic latch disengaged, high pressure fuel is delivered to the actuation port, closing the shutoff valve and stopping all fuel flow to the engines.
The foremost advantage of this type of system is that the mechanism that stops engine fuel flow is electrically and mechanically independent of the mechanism that starts it. Fewer components would be required if a single solenoid valve were used to control the pressure at the actuation port on the shutoff valve. With an electronic OR circuit, the shutdown and run commands could be reduced to a single signal that controls the state of this single solenoid valve. If all components perform as intended, this configuration functions identically to the aforementioned two-solenoid valve one, and a solenoid valve and sequencing valve are eliminated. However, should a single element fail in the single-solenoid valve system, fuel shutoff and run capability could be lost. In the two-solenoid valve system, due to isolation, at least two elements must fail in order to completely lose control of the shutoff valve, a much less probable occurrence.